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NASA and commercial spaceflight companies will soon be retuning humans to the Moon and then eventually sending them on to Mars. These distant planetary destinations will pose new risks—in particular for the health of the astronaut crews. The bulk of the evidence characterizing human health and performance in spaceflight has come from missions in Low Earth Orbit. As missions last longer and travel farther from Earth, medical risk is expected to contribute an increasing proportion of total mission risk. To date, there have been no reliable estimates of how much. The Integrated Medical Model (IMM) is a Probabilistic Risk Assessment (PRA) Monte-Carlo simulation tool developed by NASA for medical risk assessment. This paper uses the IMM to provide an evidence-based, quantified medical risk estimate comparison across different spaceflight mission durations. We discuss model limitations and unimplemented capabilities providing insight into the complexity of medical risk estimation for human spaceflight. The results enable prioritization of medical needs in the context of other mission risks. These findings provide a reasonable bounding estimate for medical risk in missions to the Moon and Mars and hold value for risk managers and mission planners in performing cost-benefit trades for mission capability and research investments.

BACKGROUND Astronauts on exploration missions may be at risk for traumatic injury and medical conditions that lead to life threatening hemorrhage. Resuscitation protocols are limited by the austere conditions of spaceflight. Solutions may be found in low‐resource terrestrial settings. The existing literature on alternative blood product administration and walking blood banks was evaluated for applicability to spaceflight. STUDY DESIGN AND METHODS A literature review was done using PubMed and Google Scholar. References were crosschecked for additional publications not identified using the initial search terms. Twenty‐seven articles were identified, including three controlled trials, six retrospective cohort analyses, 15 reviews, one case report, and two experimental studies. RESULTS Solutions to blood transfusion in austere settings include lyophilized blood products, hemoglobin‐based oxygen carriers (HBOCs), and fresh whole blood. Many of these products are investigational. Protocols for walking blood banks include methods for screening and activating donors, transfusion, and monitoring for adverse reactions. Microgravity and mission limitations create additional challenges for transfusion, including baseline physiologic changes, difficulty reconstituting lyophilized products, risk of air emboli during transfusion, equipment constraints, and limited evacuation and surgical options. CONCLUSION Medical planning for space exploration should consider the possibility of acute blood loss. A model for “floating” blood banks based on terrestrial walking blood bank protocols from austere environments is presented, with suggestions for future development. Constraints on volume, mass, storage, and crew, present challenges to blood transfusion in space and must be weighed against the benefits of expanding medical capabilities.